As an olefin-based copolymer typically including an ethylene/α-olefin copolymer or a propylene/α-olefin copolymer, there have been produced a broad range of polymers having different physical properties that are excellent in mechanical properties and cover from hard to soft ones; and regarding their use applications, the polymers are widely used covering from industrial materials for films, sheets, fibers, nonwoven fabrics, various containers, molded articles, modifiers and others down to life materials.
In general, it is known that copolymers produced through copolymerization with a comonomer such as 1-butene, 1-hexene or the like are excellent in the performance of flexibility, low-temperature impact resistance, environmental stress cracking resistance, transparency or the like, as compared with an ethylene homopolymer and a propylene homopolymer. For further improving these capabilities, it is necessary to increase the comonomer content in the copolymer while the high-molecular weight of the copolymer is kept as such. In addition, it is known that, when a complex catalyst, as typified by a metallocene catalyst, is used, then the comonomer introduced into the copolymer can be uniformly distributed therein and the above-mentioned capabilities of the copolymer can be thereby further improved.
Consequently, in producing ethylene and propylene copolymers having these capabilities, it is desired to provide a complex catalyst excellent in copolymerizability and capable of producing a high-molecular-weight olefin copolymer in a temperature range of from 50 to 300° C. that is efficient in an industrial process and capable of producing a copolymer having a high comonomer content even at a low comonomer concentration from the viewpoint of the process load. However, there is a report pointing out a problem that, in olefin copolymerization using a complex catalyst, as typified by a metallocene catalyst, the molecular weight of the polymer to be obtained decreases with the elevation of the polymerization temperature or with the increase in the comonomer content in the resultant copolymer (for example, see NPL 1).
Accordingly, for producing the olefin polymers having these capabilities, there is desired a metallocene catalyst excellent in copolymerizability in a temperature range of from 50 to 300° C. efficient in an industrial process and capable of producing an olefin copolymer having a high molecular weight.
As a metallocene having excellent copolymerizability and capable of producing an olefin copolymer having a high molecular weight, there is known a bridged bisindenyl complex having a phenyl group at the 4-position of the indenyl ring. In particular, it is reported that substituent introduction of a methyl group into the 2-position of the indenyl ring is effective for improving the molecular weight (see NPL 2, 3), and therefore, searching for a 2-positioned substituent in a bridged bisindenyl complex having a phenyl group at the 4-position thereof is kept continued for producing a polymer having a higher molecular weight.
NPL 4 reports that, in ethylene polymerization under a high-pressure condition, a complex having an iPr group can produce a polyethylene having a higher molecular weight than a complex having an Me group as the 2-position substituent thereof. PTL 1 and 2 report a complex having an α-branched alkyl substituent at the 2-position, PTL 3 reports a complex having a hetero-aromatic ring substituent at the 2-position, and PTL 4 to 7 report a complex in which the 2-positioned substituent differs between the two bisindenyl rings, all saying that the respective complexes are effective for producing high-molecular-weight olefin polymers and olefin copolymers. However, as a result of the present inventors' investigations, it has become clarified that when these catalysts are used for polymerization at a high temperature preferred from the production efficiency, then the molecular weight of the resultant polymers is insufficient.
In addition, when the structure of the 2-positioned substituent becomes more complicated, and in case where such complexes are produced on a large scale that is industrially necessary in organic synthesis, there occurs a problem in that the production cost for the complexes increases owing to the necessity of complicated synthesis routes and multi-stage synthesis routes. Consequently, it is desired to develop a novel complex capable of exhibiting more excellent performance than already-existing catalysts, and at the same time, having a simple complex structure as compared already-existing ones and capable of being synthesized easily.
Given the situation, there has been desired a metallocene compound which is easy to synthesize and which can produce a high-molecular weight olefin copolymer at a polymerization temperature and under polymerization conditions that are advantageous industrially while maintaining excellent copolymerizability, as well as a metallocene catalyst and a production method for an olefin polymer using the compound.